Processes for manufacture of high light to electrical energy conversion efficiency thin film photovoltaic cells are known to utilize a first layer of copper indium diselenide in heterojunction with one or more layers of cadmium sulfide.
For example, U.S. Pat. No. 4,335,266 discloses a method for forming a copper indium diselenide layer in two distinct regions, wherein the first region contains an excess of copper and the second region is copper deficient. Diffusion between the two layers achieves a uniform copper indium diselenide structure to reduce the formation of pure copper nodules near the copper indium diselenide surface where the cadmium sulfide layer is to be deposited. However, despite the improvements in the copper indium diselenide layer, it has still been found necessary to deposit a cadmium sulfide layer to achieve high efficiency.
“While various improvements have been made in the manufacture of copper indium diselenide CdS cells, several complications remain. For example, chemical bath deposition of cadmium sulfide is used to produce the highest efficiency devices. However, this step involves a slow wet chemical step inconsistent with an otherwise in-line dry fabrication process. Moreover, cadmium and thiourea are highly toxic materials which escalate manufacturing costs as a result of the handling and disposal of these hazardous waste materals”.
Several attempts to avoid handling complications inherent in the use of CdS are disclosed in “A ZnO/p-CuInSe2 Thin Film Solar Cell Prepared Entirely by Spray Pyrolysis,” M. S. Tomar and F. J. Garcia, Thin Solids Films, 90 (1982). p. 419–423; and “Chemical Vapor Deposited Copper Indium Diselenide Thin Film Materials Research” Final Report, March 1984, SERI/STR-211-2247. Although these publications disclose copper indium diselenide/zinc oxide heterojunction formation using zinc oxide spray pyrolysis or ion beam sputtering respectively, neither method results in an efficiency of greater than 2–3%. Accordingly, these publications do not disclose a commercially viable method for the replacement of CdS with zinc oxide in a thin film copper indium diselenide heterojunction cell.
U.S. Pat. No. 4,612,411, describes the preparation of a thin film heterojunction photovoltaic cell formed from copper indium diselenide, as a first semiconductor layer, and the formation of a two layer, zinc oxide semiconductor in heterojunction with the copper indium diselenide. The first of the two zinc oxide layers comprises a relatively thin layer (100–2000 angstroms) of bigh resistivity zinc oxide and the second comprises a relatively thick (10,000 angstroms) zinc oxide layer doped to exhibit low resistivity.
U.S. Pat. No. 5,474,939, produces a higher efficiency non-CdS cell through the application of a wet chemical deposition zinc hydroxide precipitation step. This process involves the use of a metal back contact having a first p-type semiconductor film of chemical vapor deposition (“CVD”) copper indium diselenide and a second transparent n-type semiconductor film of CVD zinc oxide on the copper indium diselenide and a thin interfacial film of transparent insulating zinc oxide, between the p-type copper indium diselenide film and the n-type zinc oxide. The interfacial zinc oxide film is prepared by chemical deposition of zinc hydroxide on the copper indium diselenide from a zinc salt solution and complexing agents comprising ammonium hydroxide or triethanolamine, to form a zinc ammonium solution complex, and annealing the deposit to covert the zinc hydroxide into the zinc oxide. While this patent uses a wet chemical deposition step of zinc hydroxide precipitate from solution to generate a thin interfacial zinc oxide layer, the devices prepared by direct deposition of a zinc oxide layer, the devices prepared by direct deposition of a zinc oxide layer on copper indium diselenide films are only 2–4% conversion efficient in spite of utilizing films capable of producing 15–17% cells.
Accordingly, there is a need in the art to obtain copper indium gallium diselenide thin film photovoltaic cells characterized by high conversion efficiency without incurring the disadvantages of: utilzing a CdS layer via wet chemistry; utilizing slow, batch processes; employing the highly toxic material of Cd, which escalates manufacturing costs as a result of handling and disposal of this hazardous waste; incurring a reduction in collected current; and forming junctions at 60–80° C.